Communicate to the reader the current way of doing things, both explicitly and implicitly. Use the idioms recommended in edge. Reorder sections to emphasize favored approaches if needed, etc. The documentation should be a model for best practices and canonical, modern Rails usage.

Documentation has to be concise but comprehensive. Explore and document edge cases. What happens if a module is anonymous? What if a collection is empty? What if an argument is nil?

The proper names of Rails components have a space in between the words, like "Active Support". ActiveRecord is a Ruby module, whereas Active Record is an ORM. All Rails documentation should consistently refer to Rails components by their proper name, and if in your next blog post or presentation you remember this tidbit and take it into account that'd be phenomenal.

Spell names correctly: Arel, minitest, RSpec, HTML, MySQL, JavaScript, ERB. When in doubt, please have a look at some authoritative source like their official documentation.

Use the article "an" for "SQL", as in "an SQL statement". Also "an SQLite database".

Prefer wordings that avoid "you"s and "your"s. For example, instead of

If you need to use `return` statements in your callbacks, it is recommended that you explicitly define them as methods.

use this style:

If `return` is needed it is recommended to explicitly define a method.

That said, when using pronouns in reference to a hypothetical person, such as "a
user with a session cookie", gender neutral pronouns (they/their/them) should be
used. Instead of:

When "true" or "false" are used as defined in Ruby use regular font. The
singletons true and false need fixed-width font. Please avoid terms like
"truthy", Ruby defines what is true and false in the language, and thus those
words have a technical meaning and need no substitutes.

As a rule of thumb, do not document singletons unless absolutely necessary. That
prevents artificial constructs like !! or ternaries, allows refactors, and the
code does not need to rely on the exact values returned by methods being called
in the implementation.

For example:

`config.action_mailer.perform_deliveries` specifies whether mail will actually be delivered and is true by default

the user does not need to know which is the actual default value of the flag,
and so we only document its boolean semantics.

An example with a predicate:

# Returns true if the collection is empty.
#
# If the collection has been loaded
# it is equivalent to <tt>collection.size.zero?</tt>. If the
# collection has not been loaded, it is equivalent to
# <tt>collection.exists?</tt>. If the collection has not already been
# loaded and you are going to fetch the records anyway it is better to
# check <tt>collection.length.zero?</tt>.
def empty?
if loaded?
size.zero?
else
@target.blank? && !scope.exists?
end
end

The API is careful not to commit to any particular value, the method has
predicate semantics, that's enough.

class Array
# Calls +to_param+ on all its elements and joins the result with
# slashes. This is used by +url_for+ in Action Pack.
def to_param
collect { |e| e.to_param }.join '/'
end
end

Using +...+ for fixed-width font only works with simple content like
ordinary method names, symbols, paths (with forward slashes), etc. Please use
<tt>...</tt> for everything else, notably class or module names with a
namespace as in <tt>ActiveRecord::Base</tt>.

When "true" and "false" are English words rather than Ruby keywords use a regular font:

# Runs all the validations within the specified context.
# Returns true if no errors are found, false otherwise.
#
# If the argument is false (default is +nil+), the context is
# set to <tt>:create</tt> if <tt>new_record?</tt> is true,
# and to <tt>:update</tt> if it is not.
#
# Validations with no <tt>:on</tt> option will run no
# matter the context. Validations with # some <tt>:on</tt>
# option will only run in the specified context.
def valid?(context = nil)
...
end

When writing documentation for Rails, it's important to understand the difference between public user-facing API vs internal API.

Rails, like most libraries, uses the private keyword from Ruby for defining internal API. However, public API follows a slightly different convention. Instead of assuming all public methods are designed for user consumption, Rails uses the :nodoc: directive to annotate these kinds of methods as internal API.

This means that there are methods in Rails with public visibility that aren't meant for user consumption.

An example of this is ActiveRecord::Core::ClassMethods#arel_table:

module ActiveRecord::Core::ClassMethods
def arel_table #:nodoc:
# do some magic..
end
end

If you thought, "this method looks like a public class method for ActiveRecord::Core", you were right. But actually the Rails team doesn't want users to rely on this method. So they mark it as :nodoc: and it's removed from public documentation. The reasoning behind this is to allow the team to change these methods according to their internal needs across releases as they see fit. The name of this method could change, or the return value, or this entire class may disappear; there's no guarantee and so you shouldn't depend on this API in your plugins or applications. Otherwise, you risk your app or gem breaking when you upgrade to a newer release of Rails.

As a contributor, it's important to think about whether this API is meant for end-user consumption. The Rails team is committed to not making any breaking changes to public API across releases without going through a full deprecation cycle. It's recommended that you :nodoc: any of your internal methods/classes unless they're already private (meaning visibility), in which case it's internal by default. Once the API stabilizes the visibility can change, but changing public API is much harder due to backwards compatibility.

A class or module is marked with :nodoc: to indicate that all methods are internal API and should never be used directly.

To summarize, the Rails team uses :nodoc: to mark publicly visible methods and classes for internal use; changes to the visibility of API should be considered carefully and discussed over a pull request first.

Feedback

You may also find incomplete content or stuff that is not up to date.
Please do add any missing documentation for master. Make sure to check
Edge Guides first to verify
if the issues are already fixed or not on the master branch.
Check the Ruby on Rails Guides Guidelines
for style and conventions.

If for whatever reason you spot something to fix but cannot patch it yourself, please
open an issue.